Vice President Biden arrived early today in Iraq for talks with the new government's leaders about the future of American troops in the country as they prepare to leave at year's end.

Biden's unannounced trip marks the first by a top U.S. official since Iraq approved a new Cabinet last month, breaking a political deadlock and jump-starting its stalled government after inconclusive parliamentary elections in March.

Iraqi officials said they expected the issue of whether to keep some U.S. forces in Iraq beyond the Dec. 31 withdrawal deadline would dominate Biden's talks with President Jalal Talabani, Prime Minister Nouri al-Maliki and Kurdish President Massoud Barzani.

Under a security agreement between Washington and Baghdad, all American troops are to leave Iraq by the end of the year. However, Iraq's top military commander has said U.S. troops should stay until Iraq's security forces can defend its borders, which he said could take until 2020.

But al-Maliki, pressured by hard-line Shiite Muslims, has signaled he wants American troops to leave on schedule. Last weekend, the influential and anti-American cleric Muqtada al-Sadr returned to Iraq after nearly four years of exile in Iran, in part to insist that the U.S. "occupiers" must leave on time or face retribution among his followers "by all the means of resistance."

The Obama administration has maintained it would leave on time unless Iraqi officials asked the United States to reconsider the security agreement and allow at least some troops to stay.

Just under 50,000 U.S. forces remain in Iraq, and American military leaders have said privately they will need to start planning in early spring on how to get them home unless told otherwise.

Keeping troops in Iraq presents a political headache for both President Obama, who is up for re-election next year and promised to end the war in his 2008 campaign, and for al-Maliki, who held onto a second term as prime minister only with al-Sadr's support.

The visit is Biden's seventh since January 2009. He arrived in Iraq after stops in Afghanistan and Pakistan, where the U.S. has refocused its efforts against al-Qaida and allied extremist groups that threaten American security.

During his visit to Islamabad, Biden urged Pakistan to pursue its fight against militants who he said pose a greater threat to the nation's sovereignty than missile strikes launched by American drone aircraft.

"Al-Qaida and its allies have found refuge in your country," Biden said during a news conference with Prime Minster Yousuf Raza Gilani. "There are those who accuse the U.S. of violating your sovereignty. But I respectfully suggest that it's extremists who violate Pakistan's sovereignty and corrupt its good name."

Hours after Biden spoke, a suicide car bomber attacked a police station and adjoining mosque in northwestern Pakistan, killing 18 people and providing a fresh reminder of America's challenges in the unstable, nuclear-armed Islamic country.